This application relates generally to turbochargers for internal combustion engines, and relates more particularly to exhaust gas-driven turbochargers having a variable vane assembly in the nozzle for the turbine in order to regulate exhaust gas flow into the turbine.
Exhaust gas-driven turbochargers employ a turbine to extract power from the stream of exhaust gas coming from an internal combustion engine and use that power to drive a compressor in order to boost the pressure of the air supplied to the cylinders. Because it is often desired to have control over the degree of boost, it is common to employ some type of variable geometry mechanism in the turbine for regulating the flow of exhaust gas to the turbine wheel. One type of variable geometry mechanism employs an array of movable vanes disposed in the turbine nozzle through which the exhaust gas is fed to the turbine wheel. The vanes can be pivoted about respective pivot axes so as to vary an effective flow area and flow direction for the exhaust gas stream into the turbine wheel.
In some such variable-vane assemblies, the vanes are pivotally mounted to a nozzle ring that forms one wall of the turbine nozzle. The opposite wall of the nozzle is sometimes formed by the turbine housing itself, or alternatively can be formed by a pipe or insert that is formed separately from the turbine housing and is mounted in the turbine housing. The present disclosure concerns primarily variable-vane assemblies of the latter type.
The clearances between the ends of the vanes and the adjacent nozzle walls, formed by the nozzle ring and the pipe or insert, are important parameters influencing the overall efficiency of the turbine. Leakage of exhaust gas between the ends of the vanes and the adjacent walls (i.e., leakage through the vane clearances) generally results in a reduction in turbine efficiency. Additionally, leakage between the pipe or insert and the turbine housing further reduces efficiency, since that leakage flow does not pass through the turbine wheel and thus is unavailable for power extraction by the wheel. The performance penalty associated with these leakage flows is particularly troublesome at low engine speeds where the exhaust gas energy is already relatively low.